Recording density of a magnetic recording apparatus having a hard disk drive has been increasing year by year, and there is a demand for increased sensitivity and density of a magnetoresistive magnetic head that is used as a read sensor. In order to satisfy such a demand, a CIP-GMR (Current in Plane-Giant Magnetoresistance) sensor having a thin non-magnetic layer sandwiched between two kinds of ferromagnetic layers, in which a current is applied in the plane, has been developed and applied as a read head. Further, in order to achieve even higher density, a read head using TMR (Tunnel Magnetoresistance) effect, in which a voltage is applied perpendicularly to a thin insulating layer sandwiched between two ferromagnetic layers, has been developed and applied. Meanwhile, as a next-generation read head capable of high-speed operation with a small resistance, a CPP-GMR (Current Perpendicular to the Plane-Giant Magnetoresistance) sensor, in which a current is applied perpendicularly to the film structure of GMR, has been researched and developed.
CPP-GMR devices generally comprise a laminate of a free layer, an intermediate layer, and a pinned layer, and the pinned layer is substantially fixed to a magnetic field to be detected by exchange coupling with an antiferromagnetic film or a like method. The magnetically secured pinned layer is stable to an external magnetic field, and only the magnetic orientation of the free layer is affected by the external magnetic field. At this time, generally, the resistance is lowest when the angle formed by the free layer and the pinned layer is 0°, and is highest when the angle is 180°. Defining the resistance change to the magnetic field angle as a magnetoresistance ratio (MR ratio=(Rmax−Rmin)/Rmin), the MR ratio of conventionally employed CPP-GMR using a CoFe alloy or an NiFe alloy is about 5% at most.
In order to increase the MR ratio of CPP-GMR devices, use of a material having large interface scattering and bulk scattering coefficients for a ferromagnetic layer of CPP-GMR has been reported. Japanese Patent No. 3607678 discloses that among Heusler alloys having a large product of bulk scattering coefficient (β)×specific resistance (ρ), particularly use of CO2MnZ (Z=Al, Si, Ga, Ge, Sn) for a free layer or a pinned layer greatly improves the MR ratio. Japanese Patent Application Pub. No. JP-A-2007-88415 discloses that use of Co—Fe—Al (50 at. %<Co<70 at. %, 10 at. %<Fe<25 at. %, 15 at. %<Al<30 at. %) for a ferromagnetic layer increases the MR ratio than in the case of Co—Fe.
Meanwhile, as another method for increasing MR ratio, Japanese Patent Application Pub. No. JP-A-2002-208744 proposes a structure having, in an intermediate layer, a current confinement layer provided with a metal path.
Incidentally, when a CPP-GMR sensor or the like is used as a magnetoresistive magnetic head, the output voltage is proportional to MR ratio×bias voltage. It thus is preferable to increase the applied voltage higher than the bias voltage (5 to 20 mV) usually used for measuring MR ratio, and use the head at a voltage that results in a high output.
However, when the voltage applied to an element is increased, the following problems will occur. Specifically, when the voltage applied to an element, i.e., a current, is increased, spin-polarized electrons are transmitted in a direction opposite to the direction of voltage application, from a pinned layer to a free layer or from a free layer to a pinned layer. At this time, the magnetization of the free layer or the pinned layer tilts due to spin torque. The problem is that because spin torque increases with an increase in the voltage, consequently, the MR ratio decreases with an increase in the bias voltage.
in particular, although CPP-GMR using Co—Mn—Z (Z=Al, Si, Ge) or a like Heusler alloy, which is a high-spin scattering material, for the pinned layer/free layer gives large MR ratio at a low bias voltage, the bias voltage dependence of MR ratio is greater as compared with CPP-GMR using Co—Fe, and there is a problem in that at a driving voltage, the resulting output voltage is smaller than expected from the MR ratio at a low bias. That is, it has been revealed that when a Heusler alloy, a high-spin scattering material, is simply used for a free layer and a pinned layer, although large MR ratio can be obtained at a low bias voltage, as a magnetoresistive element and a magnetoresistive magnetic head, a large output voltage cannot be expected.